Conventional gear tooth sensors [1] consist of a Hall effect sensor, and integrated circuit (IC) and a hard magnet, as shown in FIG. 1. The Hall IC supports two Hall sensors 12, which sense the magnetic profile of the ferromagnetic target simultaneously, but at different points, generating a differential internal analog voltage that is further processed for precise switching of the digital output signal. To achieve a high differential signal output, the two Hall probes (or sensors) are spaced so that one Hall sensor faces a field concentrating tooth and the other Hall sensor faces a gap between teeth.
Permanent magnet 13 is mounted, with either pole being to the rear of IC 11, to provide a constant magnetic bias field. When one Hall sensor momentarily faces tooth 14 while the other faces gap 15 between teeth, the gear tooth acts as a flux concentrator. It increases the flux density through the Hall probe and a differential signal is produced. As the toothed wheel turns, the differential signal changes its polarity, following the change from tooth to gap. An integrated high-pass filter regulates the differential signal to zero by means of a time constant that can be set with an external capacitor. In this way only those differences are evaluated that changed at a minimum rate. The output signal is not defined in the steady state.
An anisotropic magneto-resistance (AMR) based sensing structure has also been used as a gear tooth sensor, in which the sensing structure is similar to traditional Hall IC based gear tooth sensor except that two Hall probes are replaced by single AMR bridge 21[2], as shown in FIG. 2. Due to the high sensitivity of AMR materials, the AMR based gear tooth sensors provide extremely large output signal from AMR elements, which is stable over the rated temperature and voltage range. As a result, the AMR based gear tooth sensors feature excellent (large) air-gap performance and an extremely stable operating envelops as well as the robust reliability characteristics.
As shown in FIG. 2, the magnetic field generated by the bias magnet is influenced by the moving ferromagnetic gear tooth so AMR sensing bridge 21 detects the variation of the magnetic field component 22 within the AMR film plane. The signal output is then generated from differential signal the AMR bridge.
In this design, the “barber pole” AMR structure, shown in FIG. 3, is used to achieve linearization of the signal output, while a stabilization field is provided by a magnetic field component in the y-direction (normal to the direction of motion of the gear tooth wheel) of the permanent magnet to prevent possible AMR magnetization flipping due to external stray field. This y-component is achieved by tilting the permanent magnet away from its original perfectly perpendicular direction. As the gear toothed wheel turns, the AMR sensing element also experiences a changing magnetic field along the x-direction, so Vout-1-Vout-2, the differential voltage from this bridge, changes its polarity at the same rate as going from tooth to gap, as seen earlier in FIG. 2. Note that, in FIG. 3, the “barber pole” structure shown there comprises diagonal stripes 32 of permalloy separated by non-magnetic, but electrically conductive, metallic stripes.
Another design, proposed by K. Van Ostrand, et al. [3], uses a saturated magnetization approach. In this method, a permanent magnet is designed with a gap where the AMR bridge is located, so that the large field produced by the permanent magnet is sufficient to force the magnetizations in the AMR bridge circuit into a saturated mode in which AMR resistance no longer responds to a change in magnetic field strength; instead, it responds only to changes in magnetic field direction when a ferromagnetic gear tooth target moves past a face of the magnet. In this approach, the permanent magnet design is both complicated and expensive.